1. Field of the Invention
The present invention relates to a laser beam printer or similar tandem color image forming apparatus using an image transfer belt or an intermediate image transfer belt and more particularly to the characteristics of high-resistance backup rollers each contacting the inner surface of the belt.
2. Description of the Background Art
Today, to meet the increasing demand for high speed, advanced function color image formation, a direct image transfer type of tandem color image forming apparatus is predominant over an indirect type of color image forming apparatus using an intermediate image transfer body. The direct image transfer type of apparatus sequentially transfers toner images of different colors from a plurality image carriers arranged side by side to a sheet or recording medium being conveyed by an image transfer belt one above the other. This type of image forming apparatus, disclosed in Japanese Patent Laid-Open Publication No. 2001-324883 by way of example, includes a first to a fourth image forming station each being assigned to a particular color.
It is a common practice to output various applications edited by a personal computer or print images picked up by a digital camera in color. An image forming apparatus is therefore is used by various users not only in air-conditioned offices but also in other various environments. It follows that an image forming apparatus is required to deal with various kinds of recording media, including plain papers and coated papers, and various kinds of temperature and humidity environments. Further, an image forming apparatus compact enough to be handled by any user is desired.
The direct image transfer type of apparatus has the following problem although it is far higher in print speed than the indirect image transfer type of apparatus. In the direct image transfer type of apparatus, every time a sheet, electrostatically adhered to the image transfer belt, is passed through an image transfer nip formed in each image forming station, the sheet is charged due to separation discharge occurring between the sheet and the image carrier. The sheet is therefore charged up little by little as it advances toward the downstream image forming station. As a result, when a strong electric field is formed by an image transfer bias at the inlet of the nip of the next image forming station where the sheet is spaced from the image carrier, it is likely that a toner image carried on the sheet is scattered by pretransfer. This is particularly true when the image carrier is provided with a small diameter for reducing the overall size of the apparatus, because a nip between the image carrier and a bias applying member decreases in width.
To solve the above problem, Japanese Patent Laid-Open Publication No. 63-97976, for example, teaches a monochromatic image forming apparatus in which a press roller causes a sheet to contact a photoconductive drum before it is subject to the strong electric field of an image transfer bias. Further, the press roller is implemented as a conductive roller connected to ground in order to weaken the electric field at the inlet of an image transfer nip. However, such a conductive press roller is not directly applicable to the direct image transfer type of apparatus for reasons to be described hereinafter.
In the monochromatic apparatus taught in the above document in which a toner image is absent on a sheet when the sheet enters the image transfer nip, the press roller can be made conductive in order to weaken the electric field at the inlet of the nip as far as possible. However, in the direct image transfer type of tandem configuration, a toner image is present on a sheet when the sheet is conveyed to any one of the second and successive image forming stations. Therefore, as the sheet approaches the conductive press roller at the next image forming station, the electric field of toner on the sheet sharply decreases from infinity. Consequently, when the gap between the toner and the press roller connected to ground exceeds a discharge limit represented by the Paschen's law, discharge occurs and scatters the toner. Such toner scattering occurs in, among others, an RGB (red, green and blue) or similar bicolor line image. To solve this problem, in the direct image transfer type of tandem configuration, the roller at the inlet of the nip should not be conductive, but should preferably be provided with some resistance, i.e., insulative. More specifically, the roller should preferably be implemented as a high resistance roller.
However, when the high resistance press roller is held in contact with the image transfer belt whose volumetric resistance is as high as 1010 Ω·cm or above, frictional charging occurs between the press roller and the belt when the belt is in movement. When the resulting charge deposited on the press roller exceeds a certain limit, abnormal discharge also occurs. If a sheet is present on the image transfer belt when abnormal discharge occurs, the potential of the sheet varies from a portion subjected to the discharge to the other portion surrounding it. As a result, when image transfer is effected at the next image forming station by the application of a bias, an electric field necessary for image transfer is not attainable only at the above portion subject to the discharge, resulting in an image defect, as determined by experiments.
The image defect mentioned above refers to the local omission of an image in the form of spots and conspicuous in a halftone image, among others. The local omission of an image is apt to occur in a low temperature, low humidity environment in which the resistance of the press roller and that of the image transfer belt increase and when the amount of charge to deposit on the sheet increases. The local omission therefore frequently occurs when, e.g., an image is printed on the reverse surface of a sheet, which has been subjected to fixation and therefore noticeably lowered in water content, in a duplex print mode or when use is made of an OHP (OverHead Projector) film or similar recording medium whose volumetric resistivity is as high as 1014 Ω·cm or above.
Further, the direct image transfer type of tandem configuration has other problems to be described hereinafter. While the image transfer belt is conveying a sheet, toner images are directly transferred from the image carriers to the sheet one above the other. Therefore, when the sheet being conveyed is subject to the conveying force of a registration roller pair, fixing roller or similar conveying member other than the image transfer belt, colors are shifted from each other due to a small difference in linear velocity between the conveying member and the belt. Color shift also occurs when the sheet skews due to a small difference in vector between the direction of movement of the sheet conveyed by the registration roller pair and that of the image transfer belt.
In light of the above, there has been proposed an indirect image transfer type of tandem image forming apparatus in which a plurality of image forming units, each including a respective image carrier and a respective developing device, are arranged side by side while facing an intermediate image transfer belt. In this type of apparatus, toner images of different colors are directly transferred from the image carriers to the intermediate image transfer belt one above the other by primary image transfer, completing a four-color image on the belt. The four-color image is then transferred from the intermediate image transfer belt to a sheet by secondary image transfer. Even this type of apparatus has the same problems as the directly image transfer type of apparatus, as will be described hereinafter.
The intermediate image transfer belt to which toner images of different colors are to be directly transferred should preferably include a surface layer whose surface resistivity is as high as 1012 Ω·cm2, so that a bicolor text image, for example, is free from toner scattering ascribable to image transfer. Such high resistance, however, causes the intermediate image transfer belt to be charged by separation discharge that occurs between the belt and the image carriers at consecutive image transfer nips, resulting in toner scattering ascribable to pretransfer. This will be readily understood when the term “intermediate image transfer belt” is substituted for the term “sheet” stated earlier.
Further, the conductive press roller, connected to ground, stated previously cannot be directly applied to the indirect image transfer type of tandem configuration either. This will also re readily understood when the term “intermediate image transfer belt” is substituted for the term “sheet”.